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Der Pharma Chemica, 2015, 7(3):100-104
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ISSN 0975-413X
CODEN (USA): PCHHAX
Green condensation reaction of aromatic aldehydes with rhodanine catalyzed
by alum under microwave irradiation
T. Swathi1 and Medidi Srinivas2*
1
Department of Chemistry, SN Vanitha Maha Vidyalaya, Nampally, Hyderabad
Department of Medicinal Chemistry, Geethanjali College of Pharmacy, Cheeryal (V), Keesara (M), R.R. Dist
_____________________________________________________________________________________________
2
ABSTRACT
An efficient one pot alum catalyzed, microwave enhanced, clean process for the synthesis of 5-arylidenerhodanines
by the Knoevenagel condensation of aromatic aldehydes with rhodanine in aqueous media is described. Only 15
mole % of alum is enough in this protocol to achieve optimal yields and this protocol is fairly general and
applicable to aromatic aldehydes. The use of alum, water and MW has promising reaction response, such as short
reaction time, excellent product yields, and simple work-up of the products makes the reaction convenient, more
economic, and environmentally benign.
Key words: Microwave irradiation, Rhodanine, Alum catalyst
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INTRODUCTION
Heterocyclic chemistry is of great importance to the medicinal chemists because the steady growth of interest in
heterocyclic compounds is connected with their therapeutic activity. Further, the compounds containing the 2thioxothiazolidin-4-one ring (rhodanine derivatives) have demonstrated wide range of pharmacological activities,
which include antimicrobial [1-3], antiviral [4], antidiabetic [5] and anticancer activity [6]. Additionally, rhodaninebased molecules have been popular as small molecule inhibitors of numerous targets such as HCV NS5B protease
[7], HCV NS3 protease [8], aldose reductase [9], β-lactamase [10], UDP-N-acetylmuramase/L-alanine ligase [11],
fungal protein mannosyl transferase-1 (PMT1) [12], cathepsin-D [13], anthrax lethal factor protease [14], histidine
decarboxylase [15], JNK-stimulating phosphatase-1 (JSP-1) [16] and phosphodiesterase (PDE-4) [17]. Among the
thiazolidine derivatives, numerous compounds containing thiazolidine-2,4-dione and rhodanine have been
recognized as new potential anticancer agents. For example, GSK1059615 is a potent, reversible, ATP-competitive,
thiazolidinedione inhibitor of PI3K α [18-19].
Now-a-days one pot reactions are gaining prominence due to their environmental advantages and cost effectiveness.
Microwave mediated synthesis in organic chemistry reaching a stage of importance because of its proven ability to
accelerate even very robust transformations [20]. Biological activities associated with rhodanines and as a part of
our research interest in the development of multifunctional libraries of rhodanine, therefore it was felt worthwhile to
study these reactions in presence of alum under microwave conditions with the aim of decreasing the reaction time
and increasing the yield. Alum (KAl(SO4)2.12H2O) is a cheap, non-toxic, inexpensive, eco-friendly, and easy
handling catalyst and found use in various organic reactions such as in the synthesis of dihydropyrimidines via
Biginelli [21], coumarins [22], cis-isoquinolonic acids [23], 1,3,4-oxadiazoles [24], 1,5-benzodiazepines [25],
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T. Swathi and Medidi Srinivas
Der Pharma Chemica, 2015, 7 (3):100-104
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dibenzoxanthines [26], trisubstituted imidazoles [27], 2,3-dihydroquinazolin-4(1H)-ones [28], αβ-unsaturated acids
[29] and 5-arylidene-2,4-thiazolidinedione[30] were reported.
In continuation of our work on the synthesis of 5-arylidenerhodanines [31] using microwave irradiation, herein we
wish to report the alum catalyzed, microwave assisted synthesis of 5-arylidenerhodanines.
MATERIALS AND METHDS
All the chemicals and solvents were analytical grade and used without further purification. Melting points were
determined on open capillaries, using Boitus melting point apparatus, expressed in ºC and are uncorrected. 1H NMR
spectra of the compounds were recorded on Bruker AMX-400 MHz NMR spectrophotometer using TMS as an
internal standard and the values are expressed in δ ppm. Infrared spectra were recorded in KBr disc on Bruker
ALPHA-T FTIR spectrophotometer. The mass spectrum of the compounds were recorded either on Agilent-1100
ESI-Mass (Turbo Spray) Spectro photometer. Microanalyses were carried out with a Perkin-Elmer model-2400
series II apparatus and were within ± 0.4% of the theoretical values. Column chromatography was performed on
silica gel (230-400 mesh).
General procedure for the synthesis of 5-arylidenerhodanines (C1- C11) by Knoevenagel condensation:
A mixture of rhodanine 1 (20 mmol), respective aromatic aldehyde 2 (20 mmol) and NH2SO3NH4 (ammonium
sulphamate) was subjected to microwave irradiation (MWI) at 600 watts intermittently at 30 sec intervals for
specific time (3-6 min) (Scheme 1). Completion of the reaction was identified by TLC using silica gel-G. After
completion of the reaction, the mixture was poured onto crushed ice, and the solid that separated was isolated by
filtration, dried and recrystallized from ethanol to give desired product (C1- C11). The physical and spectral
characterization data represented in Table 1 and 2.
RESULTS AND DISCUSSION
Chemistry
As a part of the continued interest to develop efficient protocols for the synthesis of biologically active molecular
scaffolds via one-pot multicomponent reactions, herein is reported a highly efficient alum catalyzed synthesis of 5arylidene rhodanines by reacting rhodanine and aromatic aldehydes using alum as catalyst and water as solvent by
employing microwave energy (Scheme I). In order to optimize the reaction conditions, a typical reaction of
rhodanine and benzaldehyde were studied by varying catalyst concentration, solvent, MW power, temperature and
time; the results are given in Table 1. The reaction under conventional conditions using 15 mol % of alum in water
under reflux for 2 hr gave rise to 78% yield of the product 5-arylidenerhodanine C1. However, when the reaction
was subjected to microwave irradiation in domestic microwave, it gave rise to the product C1 with 94% yield in 10
min (Table 1, entry 4). In the absence of the catalyst, however, product is formed with low yield under MW
conditions (Table 1, entry 1). Changing the concentration of alum to 5, 10, 15, and 20 mol %, gave rise to 66, 71, 94,
and 90% yields of 4a respectively (Table 1, entries 2, 3, 4 and 5). Therefore, 15 mol % of alum was concluded to be
the optimum catalyst concentration to push the desired reaction to the maximum possible extent. In order to check
the effect of MW power, the model reaction using 15 mol % of alum was also undertaken at 200 W and 400 W
affording 82% and 94% yields respectively. Further enhancement in microwave power did not improve the product
yield. With a view to examine the solvent effect, the reaction was carried out in methanol and ethanol (Table 1,
entries 6, 7, 8 and 9) in addition to water. In addition, it was found that the solvent played a crucial role in this
reaction. Methanol and ethanol as solvents were also able to facilltate the Knoevenagel reaction. Water was found to
be the most suitable solvent for this reaction, due to the solubility of alum in water. Thus, 400 W microwave powers
in water were adopted as the optimum reaction condition for the subsequent reactions. Under the optimized set of
reaction conditions, a number of 5-arylidenerhodanines have been synthesized in excellent yields in 6-12 min using
a variety of aromatic aldehydes (Table 2). The purity of the compounds was checked by TLC and elemental
analyses. Both analytical and spectral data (1H-NMR) of all the synthesized compounds were in full agreement with
proposed structures. The 1H NMR spectra of the 5-arylidenerhodanines revealed the characteristic methylenic proton
in between δ 7.1 and 7.94. The spectra also showed the peaks accounting for the aromatic protons and for the
different substituents present in between the corresponding regions of the spectrum. The spectra characterization
data is given in Table 3.
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T. Swathi and Medidi Srinivas
Der Pharma Chemica, 2015, 7 (3):100-104
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Herein, we have developed an efficient and green method for one pot synthesis of 5-arylidenerhodanines has been
achieved by two component knoevanagel condensation reaction of aromatic aldehyde and rhodanine using alum as
catalyst in water. The method makes use of commercially available and inexpensive starting materials and offers
several advantages, such as avoiding toxic solvent or catalyst, simple work-up procedure, avoiding chromatographic
purification, and improved yields. Inthis methodology, the products are isolated in pure form by simple filtration and
as a result of which yield losses are avoided during workup. To investigate the generality of the reaction various
substituted and unsubstituted aromatic aldehydes were studied. Aromatic aldehydes with electron withdrawing
substituents also reacted smoothly with short reaction time and with excellent yield without any byproduct. It is
believed that many therapeutically active molecules could be synthesized by this environmentally friendly
multicomponent approach.
Scheme I
Ar
S
O
N
H
S
Alum (15 mol%)
S
+
Ar-CHO
water, MWI
O
S
C1 - C11
2a-k
1
N
H
Ar
Cl
Cl
S
N
2a
2c
2b
O
NO2
Cl
2d
2e
2f
2g
N
2h
OH
CH3
2i
2j
F
2k
Table 1. Optimization of reaction conditions for MW- assisted synthesis of C1
Entry
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
Catalyst (mol%)
--5
10
15
20
15
20
15
20
15
Solvent
MW (W)
water
400
water
400
water
400
water
400
water
400
methanol
400
methanol
400
ethanol
400
ethanol
400
water
--a: Isolate yield
Time (min)
60
10
10
10
10
10
10
10
10
10
Yield (%)a
25
66
71
94
90
52
54
58
63
78
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Der Pharma Chemica, 2015, 7 (3):100-104
_____________________________________________________________________________
Table 2. Microwave assisted synthesis of 5-arylidenerhodanines catalyzed by alum in water
Compound
C1
C2
C3
C4
C5
C6
C7
C8
C9
C10
C11
Aldehyde
phenyl
2-chlorophenyl
4-pyridyl
2,4-dichlorophenyl
4-nitrophenyl
2-thienyl
3-methoxyphenyl
2-pyridyl
4-hydroxyphenyl
4-methylphenyl
4-fluorophenyl
Time (min)
10
8
9
6
8
11
12
10
8
10
7
Yield (%)a
94
95
85
92
90
86
88
86
85
82
95
m.p. [Lit.] ºC
208 [208-210] (Ref 31 )
230 [230-232] (Ref 31)
294 [290-292] (Ref 31 )
216 [232-234] (Ref 31)
244 [240-242] (Ref 31)
252 [250-252] (Ref 31)
247 [244-246] (Ref 31)
262 [262-264] (Ref 31)
197 [196-198] (Ref 31)
244 [146-248] (Ref 31)
238 [236-238] (Ref 31)
Table 3: Spectral data of 5-arylidenerhodanines (C1-C11)
Compound
IR in KBr cm-1
C1
1234 (N-C=S), 1697 (N-C=O), 1586 (N-H), 3054 (Ar-CH)
C2
1233 (N-C=S), 1696 (N-C=O), 1589 (N-H), 3072 (Ar-CH), 849 (C-Cl)
C3
1242 (N-C=S), 1700 (N-C=O), 1596 (C=N), 3014 (Ar-CH)
C4
1227 (N-C=S), 1696 (N-C=O), 1579 (N-H), 3100 (Ar-CH)
C5
1228 (N-C=S), 1716 (N-C=O), 1597 (N-H), 3265 (Ar-CH), 1505
(N=O, asymmetric), 1339 (N=O, symmetric)
C6
1186 (N-C=S), 1680 (N-C=O), 1578 (N-H), 3056 (Ar-CH), 665 (C-S)
C7
1213 (N-C=S), 1685 (N-C=O), 1584 (N-H), 3154 (Ar-CH), 1166 (OCH3)
C8
1233 (N-C=S), 1715 (N-C=O), 1596 (C=N), 3113 (Ar-CH)
C9
1173 (N-C=S), 1712 (N-C=O), 1603 (N-H), 3160 (OH)
C10
1185 (N-C=S), 1686 (N-C=O), 1578 (N-H), 3039 (Ar-CH)
C11
1235 (N-C=S), 1694 (N-C=O), 1580 (N-H), 3023 (Ar-CH), 1120 (C-F)
1
H-NMR (CDCl3 / TMS) δ ppm
7.65(1H, s, =CH), 7.50-7.67 (5H, Ar-H), 13.85 (1H, s,
N-H).
7.67 (1H, s, =CH), 7.53-7.77 (4H, Ar-H), 13.96 (1H, s,
N-H).
7.58 (1H, s, =CH), 7.54-8.74 (4H, Ar-H).
7.85 (1H, s, =CH), 7.54-7.86 (3H, Ar-H), 13.99 (1H, s,
N-H).
7.75 (1H, s, =CH), 7.75-8.36 (4H, m, Ar-H), 13.98
(1H, s, NH).
7.94 (1H, s, =CH), 7.32-8.10 (3H, Heteroaryl-H),
13.80 (1H, s, N-H).
7.4 (1H, s, =CH), 7.08-7.63 (4H, m, Ar-H), 3.82 (3H,
s, OCH3), 13.83 (1H, s, N-H).
7.67 (1H, s, =CH), 7.42-8.8 (4H, Heteroaryl-H), 13.6
(1H, s, N-H).
7.7 (1H, s, =CH), 7.48-7.83 (4H, m, Ar-H), 13.91 (1H,
s,N-H).
7.57 (1H, s, =CH), 7.36-7.62 (4H, m, ar-H), 2.37 (3H,
s, CH3), 13.81 (1H, s, N-H).
7.65 (1H, s, =CH), 7.38-7.70 (4H, m, ar-H), 13.82 (1H,
s, NH).
CONCLUSION
In the present study we have demonstrated a simple, efficient and cleaner strategy for the synthesis of 5arylidenerhodanines by knoevanagel condensation of different aromatic aldehydes with rhodanine in presence of alum
in water by employing microwave irradiation power. Moreover, the catalyst used is easily available, inexpensive, nontoxic, eco-friendly, facile work-up, which makes the reaction convenient, more economic, and environmentally
benign. The products are obtained in much shorter time as compared to the conventional method with increase in the
yield from 4-23%. The quality of products is also found to be better as compared to the conventional method.
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